1. Field of the Invention
The present invention relates to an optical disc apparatus and a method for reproducing information, being favorably applicable to an optical disc apparatus adapted to reproduce holograms recorded on optical discs, for example.
2. Description of the Related Art
An optical disc apparatus according to related art, in which a light beam is irradiated to an optical disc, such as a CD (compact disc), a DVD (digital versatile disc) and a Blue-ray Disc (registered trademark, hereafter referred to as BD), and the reflected light beam thereof is read so as to reproduce information, has been used widely.
In such an optical disc apparatus according to related art, information is recorded by irradiating a light beam to the optical disc and by changing the local reflectivity or the like of the optical disc.
With respect to the optical disc, it is known that the size of the optical spot formed on the optical disc is approximately given by λ/NA (λ: wavelength, NA: numerical aperture) and that the resolution is proportional to this value. For example, the BD that is capable of recording approximately 25 [GB] of data on an optical disc of 120 [mm] in diameter is detailed in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39,756 (2000) (Non-patent Document 1).
An optical disc is designed so as to record various contents, such as audio contents and video contents, or a variety of information, such as various data for computers. In particular, in recent years, the amount of information has increased to enable higher video resolution and higher audio quality. Furthermore, the number of contents to be recorded on a single optical disc is demanded to be increased. For these reasons, the optical disc is demanded to have larger capacity.
To fulfill such demands, a method has been proposed in which recording layers are overlaid inside a single optical disc to increase its recording capacity (for example, refer to I. Ichimura et al, Technical Digest of ISOM′ 04, pp 52, Oct. 11-15, 2005, Jeju Korea (Non-patent Document 2)).
On the other hand, an optical disc apparatus that uses holograms as a recording method for recording information on optical discs has also been proposed (for example, refer to R. R. McLeod et al., “Microholographic multilayer optical disk data storage,” Appl. Opt., Vol. 44, 2005, pp 3197 (Non-patent Document 3)).
For example, as shown in FIG. 1, in an optical disc apparatus 1, a light beam from an optical head 7 is once condensed and focused inside an optical disc 8 made of photopolymer or the like, the refraction index of which changes depending on the intensity of the light beam irradiated thereto. Then, the light beam is condensed at the same focus position once again in the opposite direction using a reflection device 9 provided on the back side (the lower side in FIG. 1) of the optical disc 8.
In the optical disc apparatus 1, a light beam, i.e., a laser light beam, is emitted from a laser 2, the wave of the light beam is modulated using an acoustooptical modulator 3, and the light beam is converted into a parallel light beam using a collimator lens 4. Furthermore, the light beam is transmitted through a polarization beam splitter 5 and converted from a linearly polarized light beam into a circularly polarized light beam using a quarter wavelength plate 6, and then enters the optical head 7.
The optical head 7 is designed so as to be able to record and reproduce information. The light beam is reflected by a mirror 7A, condensed by an objective lens 7B and irradiated to the optical disc 8 that is rotated using a spindle motor (not shown).
At this time, the light beam is once focused inside the optical disc 8 and then reflected by the reflection device 9 disposed on the back side of the optical disc 8. The reflected light beam is condensed at the same focus inside the optical disc 8 from the back side thereof. The reflection device 9 has a condensing lens 9A, a shutter 9B, a condensing lens 9C and a reflecting mirror 9D.
As a result, as shown in FIG. 2A, a standing wave is generated at the focus position of the light beam, and a recording mark RM is made. The recording mark is formed of a hologram having a small optical spot size and wholly having a shape in which two cones are connected such that the vortexes thereof are made contact with each other. The recording mark RM is thus recorded as information.
In the optical disc apparatus 1, when multiple recording marks RM are recorded inside the optical disc 8, the optical disc 8 is rotated, and the recording marks RM are disposed along concentric or spiral tracks, whereby one mark recording layer is formed. Furthermore, by adjusting the focus position of the light beam, it is possible to record the recording marks RM such that multiple mark recording layers are overlaid.
Hence, the optical disc 8 has a multi-layer structure having multiple mark recording layers therein. For example, as shown in FIG. 2B, the distance p1 (mark pitch) between the recording marks RM is 1.5 [μm], the distance p2 (track pitch) between the tracks is 2 [μm], and the distance p3 between the layers is 22.5 [μm].
Furthermore, in the optical disc apparatus 1, when information is reproduced from the optical disc 8 on which the recording marks RM are recorded, the shutter 9B of the reflection device 9 is closed so that the light beam is not irradiated from the back side of the optical disc 8.
At this time, in the optical disc apparatus 1, a light beam is irradiated to a recording mark RM inside the optical disc 8 using the optical head 7, and a reproduction light beam generated from the recording mark RM enters the optical head 7. This reproduction light beam is converted from a circularly polarized light beam into a linearly polarized light beam using the quarter wavelength plate 6 and then reflected by the polarization beam splitter 5. Furthermore, the reproduction light beam is condensed by a condensing lens 10 and irradiated to a photodetector 12 via a pinhole plate 11.
At this time, in the optical disc apparatus 1, the photodetector 12 detects the amount of the reproduction light beam, and information is reproduced on the basis of the result of the detection.